The present invention relates to a multi-piece solid golf ball having an improved xe2x80x9cclickxe2x80x9d and xe2x80x9cfeelxe2x80x9d when hit with a golf club.
A variety of multi-piece golf balls, including three-piece and four-piece balls, have been developed over the past few years in order to improve ball performance.
Most of these golf balls have a center core of about 30 mm in diameter that is made of a rubber-based material to maintain the resilience of the golf ball. The rubber center core exerts a large influence on ball performance when the ball is hit. Balls designed for golfers with a high golf club head speed generally have a hard feel when hit, while balls designed for low head speed golfers have too soft a feel. Hence, there has remained substantial room for improvement. That is, balls containing a soft center core have a soft feel while balls containing a hard center core have a hard feel to the full spectrum of golfers, from players of ordinary skill to skilled amateurs and professional golfers.
The clicking sound made when a golf ball is hit, which is an important feature of the ball, is also strongly affected by the center core. Again, for the most part, balls containing a soft center core have a dull xe2x80x9cclickxe2x80x9d when hit, whereas balls containing a hard center core have a high-pitched xe2x80x9cclick.xe2x80x9d
Therefore, an object of the present invention is to provide a solid golf ball having a good feel and an agreeable click.
It has been found that the feel of a golf ball can be varied and the click improved by dividing the solid core into a center core and an outer core and optimizing each of these two layers. More specifically, it has been found that reducing the size of the center core to a diameter of 3 to 20 mm, which is smaller than in conventional golf balls, provides an unprecedented click and feel, particularly when deformation of the ball is large, without adversely affecting the resilience and other key characteristics of the ball.
Moreover, although it is conventional practice to form the center core of a rubber-based material to maintain the ball""s resilience, given the limited hardness achievable with rubber materials and the loss in productivity encountered when such materials are used to form small-diameter center cores, a center core composed primarily of a resin provides certain advantages. While the use of a resin sphere does give a center core having a lower resilience than one made of a conventional rubber material, the decline in the resilience of the ball as a whole is minimized by setting the diameter of the center core within the above-indicated range of 3 to 20 mm. The degree of this decline in the ball""s resilience varies with the diameter of the center core, becoming larger as the diameter of the resin center core increases. The specific gravity G and the diameter d in millimeters of the center core are made to satisfy the relationship:
Gxe2x89xa70.014d+0.929. 
That is, a center core having a larger diameter has a higher specific gravity. In general, adding a filler to a certain material to increase its specific gravity has the effect of lowering its resilience. When the center core is formed so that the specific gravity rises with increasing diameter, it becomes possible in turn to lower the specific gravity of the outer core and thus compensate for the low resilience of the center core. As long as the center core is given a small diameter and the specific gravity of the center core is suitably adjusted in accordance with its size, the center core can be made of resin without compromising the performance of the ball as a whole.
It has also been found that when the Shore D hardness of the center core is set within a relatively hard range of 40 to 95 and the innermost layer of the outer core is formed so as to be at least 10 Shore D units softer than the center core, the resulting soft core containing a small, hard center has the overall effect of imparting a soft, yet solid feel and an appropriately high-pitched click to the ball when hit with a golf club.
Accordingly, the present invention provides a solid golf ball comprising a solid core and a cover that encloses the solid core, the solid core having a multiplayer construction which includes a center core. The center core is composed primarily of a resin and has a diameter d of 3 to 20mm and a specific G of 0.90 to 1.50 which satisfy the condition: Gxe2x89xa70.014[a]d+0.929. The center core has a Shore D hardness at the surface thereon of 40 to 95, and the outer core has a Shore D hardness at the innermost layer portion thereof which is at least 10 units lower than the Shore D hardness at the surface of the center core.
The hardness of the overall ball can be optimized by setting the deflection of the solid core under a static load of 100 kg within a range of 2.2 to 4.3 mm. Preferably, the innermost layer of the outer core is composed primarily of resin or a mixture of resin and rubber.
By thus having the solid core of a golf ball contain a small, hard center core made of resin, there can be obtained a golf ball having an unprecedented feel and an agreeable click that is not too low in pitch.